1. Field of the Invention
This invention relates to a single lens having an aspherical surface, and in particular to a single lens having both aspherical surfaces in which NA is of the order of 0.35.
In recent years, optical disks such as video disks and compact disks as optical memories have been widely used as information recording carriers of great memory capacity.
Attention has also been paid to optical cards having advantages such as a great memory capacity and good portability as optical information carriers similar to optical disks.
In order to record information highly densely on a recording carrier of this kind of reproduce recorded information accurately, a high imaging performance is required of an optical system used in an information recording-reproducing apparatus. For example, a resolving power of the order of 1-2 .mu.m is required for an objective lens and therefore, aberration correction must be effected so that the objective lens may have NA of 0.45 or greater and have an imaging ability to the order of the limit of diffraction.
Also, in recent years, the beam of a semiconductor laser (hereinafter abbreviated as LD) used as a light source for a recording-reproducing device such as an optical disk or an optical card is a divergent light, and adoption is made of a construction in which such beam is made into a parallel beam by a collimator lens and this parallel beam is condensed to 1-2 .mu.m by an objective lens as described above. At this time, aberration correction must be effected so that the collimator lens may have NA as high as the order which can efficiently utilize the emitted light of LD and may make the light from the light emitting portion of LD of several-tenths of .mu.m.times.several .mu.m parallel, in other words, may have an imaging ability of the order of the limit of diffraction. If the collimator lens is inferior in its imaging ability, even if the resolving power of the objective lens is sufficient, the performance as the whole optical system is low and preferable recording and reproduction of information cannot be accomplished.
Further, in these information recording-reproducing apparatuses, a head unit including the above-described optical system is moved relative to the information recording carrier to thereby effect the access of information and therefore, it is desirable that each constituent unit be made compact and light in weight to the utmost.
From these viewpoints, the development of an aspherical single lens for use as an objective lens have recently been active, and various techniques are disclosed, for example, Japanese Laid-Open Patent Application No. 57-201210, Japanese Laid-Open Patent Application No. 58-68711, Japanese Laid-Open Patent Application No. 59-23313, Japanese Laid-Open Patent Application No. 59-26714, Japanese Laid-Open Patent Application No. 61-2117, Japanese Laid-Open Patent Application No. 61-11715, Japanese Laid-Open Patent Application No. 61-56314, U.S. Patent Nos. 4,027,952, 4,449,792, 4,571,034, 4,743,093, etc.
However, the aspherical single lenses shown in the aforementioned publications are designed to the objective specification for an optical disk, and are very unsuitable when they are used for an information recording carrier in which the protective layer covering the information recording surface of an optical card or the like is thin as compared with the protective layer of an optical disk, or when they are used as collimator lenses. That is, the thickness t of the transparent protective layer for which the lenses shown in the above-mentioned publications can be suited is of the order of 0.26F-0.28F. In contrast, the thickness t of a protective flat glass plate mounted on the ordinary LD is about 0.25-0.35 mm, and the focal length F of a lens which is appropriate therefor is 0.89-1.35 mm. In this case, however, there are the disadvantages that the radius of curvature becomes too small and the manufacture is very difficult and that the area having a good imaging characteristic which can be regarded as the limit of diffraction (the image height) is very narrow, and this is hardly practical. As what makes up for these disadvantages, there is conceivable a construction in which the thickness of the protective layer is used with a parallel flat plate for correction interposed, but such construction is against the desire for the improved performance by making the optical system compact and light in weight and the reduced cost resulting from a decrease in the number of parts, and is not preferable.
Also, as an aspherical single lens for a collimator, there is shown, for example, in Japanese Laid-Open Patent Application No. 60-121412, an aspherical single lens which is directed chiefly to the use for reproduction and in which NA is 0.2 or less and which is designed for the thickness t=0.0176F of the transparent protective layer, but particularly as a collimator lens recently used for recording or for both recording and reproduction, such an aspherical single lens is not preferable because the effective efficiency of the quantity of emitted light of LD is bad for NA=0.2. Further, in the case of an objective lens having NA of 0.45 or greater, the focal length of a collimator lens for which the spacing between the surface of the information recording carrier and the objective lens is secured sufficiently to prevent the contact therebetween and for creating a parallel light having a sufficient cross-section in which no eclipse is caused for the movement of the objective lens in a direction perpendicular to the optic axis thereof, such as auto tracking, is in inverse proportion to NA and therefore, a collimator lens of high NA is preferable also in making the distance (W.D.) between the collimator lens and the LD small.
Further, in recent years, there has been proposed a multibeam optical head provided with a plurality of parallel light sources such as LD arrays and using as a plurality of spots the conjugate images of the plurality of light sources formed on the surface of a recording carrier by an optical system. Such technique is disclosed, for example, in Japanese Laid-Open Patent Application No. 57-60697, Japanese Laid-Open Patent Application No. 58-220247, etc. The uses of the plurality of spots shown in these publications are various, but any of them need be imaged to a diameter of 1 to several .mu.m. In this construction, a plurality of off-axis beams or an off-axis beam and an on-axis beam are used at a time and therefore, the angle-of-view imaging characteristic of the lens used on the same focal plane becomes more severe than that of the lens heretofore used.
In the conventional lens, the off-axis use has been little devised due basically to the deviation or the like of the axis during mounting because the deterioration of the imaging by the focus movement=curvature of image field+astigmatism caused by the difference in angle of view is absorbed by an auto focusing mechanism or the adjustment during mounting.